MOSCOW -- Muscovites who dropped their pants for a day to share a frivolous No Pants Subway Ride with the world may have risked more than just their modesty.

Russian officials are hinting at possible prosecutions for the bare-legged stunt -- if only they can determine what, if any, laws or public regulations were broken during the weekend happening.

Begun in New York City in 2002, No Pants Subway Ride is a lighthearted flash mob that encourages fellow pranksters around the world to take to the tubes pantless but otherwise dress normally and try to behave normally for comic effect. It has since spread to dozens of cities around the world, including Amsterdam, Madrid, Melbourne, Mexico City, Tel Aviv, Vienna, and reportedly even Istanbul.

It hit Moscow for the first time on January 10, when temperatures in the Russian capital topped out at around minus 12 degrees Celsius.

Now, Interfax has quoted a police source as saying that law-enforcement officers are conducting a preliminary investigation to establish whether the stunt constituted a breach of the administrative code -- or even a crime.

Vladimir Putin has frequently employed ostensibly Russian national values and traditions since returning to the presidency in 2012, including to mute political dissent and stigmatize cooperation with foreigners. During the same period, laws have been enacted to stifle public protests through tough sentences for even minor demonstrations of political opposition.

Moscow's police are said to be searching for some young men and women who, like these people, posted photographs via social networks like VKontakte of themselves in underwear on the subway in the capital.

Officers have "discovered several photographs showing a group of six people without trousers in the carriage of a train on the Zamoskvoretskaya line of the metro," the agency reported.

The director of Moscow's subway, the Metro, Dmitry Pegov, condemned it as a blatant breach of transport regulations that was doubtless "unpleasant for the other passengers." He cited his Soviet upbringing and urged passengers to "be respectful of the rules of using the [subway]."

In comments to Lenta.ru, Communist Party official Sergei Malinkovich bristled at the prank, expressing doubt that "people would do this for free" and saying it must have been "initiated" and "paid for from abroad."

The Huffington Post wrote that this year's No Pants Subway Ride was staged in more than 60 cities, with tens of thousands of participants.

Moslenta, an online news resource covering Moscow news, quoted one person who took part in the action as saying he was not worried about the official reaction.

"I couldn't care less," the participant said. "I just spent some time having fun and that's it."